The instant invention relates to the manufacturing of semiconductor integrated circuits.
In the manufacturing processes of semiconductor integrated circuits, one of the steps usually provided for consists in establishing metal contacts or metal silicide contacts on the apparent silicon layers, the remaining part of the wafer being masked by silicon oxide.
More particularly, it has been proved advantageous to use a refractory metal and to provide for processes permitting to localize the formation of a metal silicide on the only places where the silicon is apparent.
U.S. Pat. No. 4,619,038 describes a process for the localized deposition onto a substrate of a silicide of a metal having a high melting point such as titanium. This patent indicates as an example that use can be made of a structure such as the one illustrated in FIG. 1 comprising a MOS transistor formed on a silicon substrate 1 within an area enclosed by a field oxide 2. This transistor comprises a drain region 3 and a source region 4 separated by a channel region covered with a gate comprising a thin oxide 5 coated with a polycristalline silicon layer 6 laterally insulaed by silicon oxide spacers 7. This patent proposes to position such a structure into a chamber comprising as a reactive gas a titanium halogenide and hydrogen. Thus, a titanium silicide 8 is formed, as shown in FIG. 2, on the apparent silicon surfaces. Since reactive gases do not contain any silicide, it is evident that silicide can be formed only from the silicon comprised within the areas 3, 4 and 6. Thus, a certain penetration of titanium silicide occurs into the silicon areas. This penetration presents a drawback because it reduces the thickness of the implanted silicon areas 3 and 4 and the thickness of the polycrystalline crystalline silicon area of gate 6. This makes it necessary to initially oversize those layers or may result in a breakdown. On the other hand, it will be noted that, according to this process, the depostion is carried out at a temperature of about 700.degree. to 1,000.degree. C., which may also constitute a drawback on account of the dopant redistributions and diffusions this is liable to entail.
Another drawback of the methods according to the prior art for locally forming the metal silicide is very schematically illustrated in relation with FIGS. 3A, 3B, 3C. As shown in FIG. 3A, when a silicon area is to be bared inside an oxide mask 11, some residues of silicon oxide 12 and/or a very thin layer of native silicon oxide unavoidably remain, for example having a thickness of a few molecules. If a depostion or a selective growth of a metal silicide is then carried out, this silicide is initially developed within the oxide-free nucleation areas, since this deposition is precisely carried out in circumstances when it is selective with respect to SiO.sub.2. Therefore, nucleation areas 13 such as those illustrated in FIG. 3B are initially found. It has also been assumed in FIG. 3B that, simultaneously to the formation of the nucleation areas, a method permitting to eliminate the oxide areas 12 has been used. As a result, as shown in FIG. 3C, once the growth or the deposition is completed, the upper surface (and possibly the lower surface) of the silicon layer presents roughnesses, which consitutes a drawback for the subsequent processes. This drawback, though not mentioned in U.S. Pat. No. 4,619,038, unavoidably appears.
U.S. Pat. No. 4,501,769 describes a selective deposition process of a silicide of a refractory metal such as molybdenum, tungsten or tantalum onto silicon.
This patent proposes to use as a reactive gas a composite comprising a metal halogenide, hydrogen or an hydrogen halogenide and a silicon hydrogenated halogenide, that is, for example a composite TaCl.sub.5, H.sub.2 or HCl, and SiH.sub.2 Cl.sub.2. However, the problems inherent to the consumption of silicon on which it is desirable to form the metal silicide are not entirely solved by this process. Indeed, the preferential temperatures that are mentioned are close to 850.degree. C. and, at those temperatures, a consumption of the substrate silicon unavoidably occurs. Even though, as suggested by the claims of this patent, it was possible to operate at lower temperatures, the problem inherent to the silicon consumption might be solved but the problem caused by the initial deoxidation of the native silicon oxide and of the resulting roughnesses would still remain unsolved.